The embodiments of the present disclosure provide an array substrate, a preparation method thereof, and a display device. The preparation method of an array substrate comprises: forming the active layer, a gate insulating layer, the gate metal layer and the patterned photoresist sequentially on a substrate; forming a gate electrode transition pattern by etching a gate metal layer via a patterned photoresist, using a wet etching process and a dry etching process sequentially; and doping an area of the active layer not sheltered by the gate electrode transition pattern with ions to form a heavily doped area of the active layer.